Front/side kick to the kneecap...effective?

What do you guys think of such a technique? I'm not referring to your typical roundhouse leg kick, but a direct kick on the knee cap causing it to bend backwards. I have yet to use it in an actual fight but based on sparring experience, it seems to be more of a distraction than anything else, setting up a punch. However, based on the typical feeling of getting hit in the kneecap, it seems that if you were to set it up and actually get a direct hit it could cripple your opponent quite quickly.

If the leg is straight so that you can hyperextend it with a kick, it would be extremely nasty. However, this is also extremely improbable; I can't imagine anyone, whether they know how to fight or not, sticking their leg out straight for you to stomp on. Why on Earth would they?

If the leg is bent, my guess is that it wouldn't do all that much and there are better places to kick. I do not speak from experience, mind, but it's certainly not the first thing I'd worry about.

Not much? The point of the post was to point out that getting the straight leg for a hyperextending kick is improbable. The second part was merely a duly-qualified side note to avoid leaving the primary question entirely disregarded. We might also suppose that since the knees, legs bent, are generally used to hit things, they are not among the most fragile parts of the body.

Well, my question wasn't directed at a straight legged opponent, but rather one in fighting stance. I was wondering if a kick with a moderate amount of power to the knee of an opponent in a typical fighting stanc (i.e. bent knee) would be effective or not.